Imaging Functional Connectivity in Visual Cortex The mammalian visual cortex is necessary for vision and has been used over the decades as a model system for the rest of the cerebral cortex. In spite of this, little is known about its microcircuits, and one could argue that to really understand what the visual cortex does one needs to open the black box and decipher them. Moreover, the visual cortex is composed of many different cell types, and, it is likely that each cell type has a particular circuit function. In the last cycle of the award, using a novel two-photon uncaging technique, we studied the GABAergic interneurons and discovered that two of their major subtypes, PARV and SOM positive cells, are connected in a very dense pattern to cells in their vicinity, often making synaptic connections with every single neighboring neuron. This was surprising since it implies that interneurons connect promiscuously, as if their function was not specific. We now propose to test if this is the case, by examining the input and output connectivity of the four major subtypes of interneurons in V1 (PARV, SOM, VIP and chandeliers). Using two-photon photoactivation with a novel caged compound and a novel channelrhodopsin, and transgenic mouse strains that label subtypes of interneurons specifically, we will map connections to and from interneurons in a systematic fashion, in order to arrive at the inhibitory circuit blueprint of mouse V1. In the last aim, using a novel SLM two-photon microscope, we will image the responses of these four subtypes of interneurons in anesthetized and awake animals, under spontaneous activity and visual stimulation. These data will be computationally analyzed to search for spatiotemporal patterns of activity indicative of specific subcircuits. Our work will provide a systematic description of the anatomical and functional connectivity of cortical interneurons, one that will enable to examine whether their function is specific or not and to understand how they regulate the activity of the visual cortex. Given the key role that interneurons appear to play in developmental plasticity, the generated knowledge could also help to understand the pathophysiology of developmental deficits in vision and to design novel therapeutic strategies to treat amblyopia.